I am attempting to remove the cylinder head
from a 4.2L Vortec 6-cylinder engine in a 2005
GMC Envoy. When I started to remove the
head bolts, three out of the six I attempted
broke off. At that point, I decided to stop. Am
I doing something wrong? Is there a special
procedure for removing these bolts? And do I
need to replace all of them when I finally get
the last one out?
John Stepp
Tucson, AZ
Like most recent engines, the Vortec is
equipped with torque-to-yield (TTY) head
bolts. You’ve probably installed bolts like these
before. Instead of specifying a torque value
for bolt installation, the manufacturer requires
that these bolts be tightened using a torque
plus angle procedure. After the bolt is tightened
to the initial torque value, it’s tightened
an additional number of degrees to reach the
final value.
Tightening bolts in this manner brings them
very close to their yield points. The yield point
is when a bolt begins to stretch or, if taken
even further, breaks. Slightly stretching bolts
gives them the resilience they’ll need to keep
the gasket properly compressed over thousands
of heating and cooling periods in the engine’s
service life.
As you might imagine, a bolt that’s been
stretched (even slightly) is going to be more
fragile than a bolt that has been tightened but
not stretched. When it comes time to remove
TTY, special precautions must be taken. GM
issued a technical service bulletin on this topic
on Oct. 21, 2008 (No. 05-06-01-026B). It applies
to a wide range of GM models, including:
2004-07 Buick Rainier
2002-09 Chevrolet TrailBlazer and GMC Envoy
2004-09 Chevrolet Colorado and GMC Canyon
2002-04 Oldsmobile Bravada
2006-09 HUMMER H3
2005-09 Saab 9-7X
These models are equipped with 2.8, 2.9,
3.5, 3.7 or 4.2L Vortec inline engines with VINs
8, 9, 6, E, S and RPOs LK5, LLV, L52, LLR,
LL8. All of these engines have TTY head bolts
and may also have TTY main bearing cap bolts.
Over time, the head bolt threads may seize
to the corresponding engine block internal
threads. When you attempt to remove the bolt,
it breaks at its weakest point before the seized
threads break free. GM’s solution to this problem
is remarkably simple. Before attempting to
remove the bolts, use an appropriately sized
punch and hammer to rap on the head of each
head bolt. The vibration should free the seized
threads and assist in successful removal. Do
not use air tools of any kind during your removal
attempt. Using hand tools only, carefully
but confidently remove the remaining bolts.
In a perfect world, that would be the end of
the story. But despite careful preparation and
your best efforts, some head bolts still may
break. The strength of these bolts may have
been compromised by the torque-to-yield procedure
and they just weren’t strong enough to
make the return trip out of the engine. Don’t
despair; GM offers an additional lifeline.
source: http://www.motor.com/magazine/pdfs/112009_02.pdf
from a 4.2L Vortec 6-cylinder engine in a 2005
GMC Envoy. When I started to remove the
head bolts, three out of the six I attempted
broke off. At that point, I decided to stop. Am
I doing something wrong? Is there a special
procedure for removing these bolts? And do I
need to replace all of them when I finally get
the last one out?
John Stepp
Tucson, AZ
Like most recent engines, the Vortec is
equipped with torque-to-yield (TTY) head
bolts. You’ve probably installed bolts like these
before. Instead of specifying a torque value
for bolt installation, the manufacturer requires
that these bolts be tightened using a torque
plus angle procedure. After the bolt is tightened
to the initial torque value, it’s tightened
an additional number of degrees to reach the
final value.
Tightening bolts in this manner brings them
very close to their yield points. The yield point
is when a bolt begins to stretch or, if taken
even further, breaks. Slightly stretching bolts
gives them the resilience they’ll need to keep
the gasket properly compressed over thousands
of heating and cooling periods in the engine’s
service life.
As you might imagine, a bolt that’s been
stretched (even slightly) is going to be more
fragile than a bolt that has been tightened but
not stretched. When it comes time to remove
TTY, special precautions must be taken. GM
issued a technical service bulletin on this topic
on Oct. 21, 2008 (No. 05-06-01-026B). It applies
to a wide range of GM models, including:
2004-07 Buick Rainier
2002-09 Chevrolet TrailBlazer and GMC Envoy
2004-09 Chevrolet Colorado and GMC Canyon
2002-04 Oldsmobile Bravada
2006-09 HUMMER H3
2005-09 Saab 9-7X
These models are equipped with 2.8, 2.9,
3.5, 3.7 or 4.2L Vortec inline engines with VINs
8, 9, 6, E, S and RPOs LK5, LLV, L52, LLR,
LL8. All of these engines have TTY head bolts
and may also have TTY main bearing cap bolts.
Over time, the head bolt threads may seize
to the corresponding engine block internal
threads. When you attempt to remove the bolt,
it breaks at its weakest point before the seized
threads break free. GM’s solution to this problem
is remarkably simple. Before attempting to
remove the bolts, use an appropriately sized
punch and hammer to rap on the head of each
head bolt. The vibration should free the seized
threads and assist in successful removal. Do
not use air tools of any kind during your removal
attempt. Using hand tools only, carefully
but confidently remove the remaining bolts.
In a perfect world, that would be the end of
the story. But despite careful preparation and
your best efforts, some head bolts still may
break. The strength of these bolts may have
been compromised by the torque-to-yield procedure
and they just weren’t strong enough to
make the return trip out of the engine. Don’t
despair; GM offers an additional lifeline.
source: http://www.motor.com/magazine/pdfs/112009_02.pdf
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